T cell CD5 and CD6 transmembrane glycoproteins are structurally related to one another and are encoded by genes that are closely linked in the genome. Absence of CD5 in mice confers hyperresponsiveness on thymocytes and T cells to signaling through their antigen receptors. One of the goals of the research proposed in this application is to determine the mechanism of this negative regulation. For this purpose, we will use an in vitro assay that informs on the capacity of mutant CD5 molecules to inhibit T cell receptor signaling. The experiments will attempt to determine which parts of the CD5 molecule are necessary for the negative regulatory effect, and they will also identify the interacting proteins that relay the inhibition. Related experiments will address the immunoregulatory roles of the CD6 molecule and will attempt to determine how alternative splicing of the RNA encoding CD6 might affect its function at different stages of I cell development. Using mouse mutants that lack expression of CD5, CD6 or both molecules, we will attempt to determine the functions of the molecules in vivo. These experiments will involve the analysis of I cell receptor transgenic mice, and also model autoimmune and infectious systems. Preliminary data suggest that CD5 and CD6 influence the outcome of thymocyte development, the course of experimental allergic encephalomyelitis and the host response to bacterial infection. Through these in vivo experiments, we hope to understand the significance of the novel regulatory roles performed by CD5 and CD6 during autoreactive and pathogen-directed immune responses. Such an understanding may eventually lead to the development of therapeutic approaches that specifically target these molecules to regulate deleterious immune responses.